Hi all! After A LOT of searching, I came across a platform specializing in RSVP management that can be embedded into Squarespace using an embed block. The site is https://rsvpify.com/ and here is a helpful article on their site that shows you how to do it https://help.rsvpify.com/en/articles/1222544-how-do-i-set-up-my-embed-to-work-with-squarespace. I'm sure this is too late for the previous people looking but hopefully helpful for people in the future. I was looking specifically for wedding RSVPs which require grouping of attendees etc but it seems like this can be used for all manner of RSVP events. Hope it helps.